This invention relates to a method for recording a phase change optical recording medium and an optical recording medium adapted for use in such method.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is irradiated to the recording layer to change its crystalline state whereupon a change in reflectivity by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
Such phase change recording layer is typically produced from a material of chalcogenide system such as Ge—Te system and Ge—Sb—Te system since the recording layer produced by using such material provides substantial difference in reflectivity between the crystalline and amorphous states as well as a relatively stable amorphous state.
When information is recorded on the phase change optical recording medium, the entire recording layer is first brought into crystalline state, and then, a laser beam of high power level (recording power level) is applied so that the recording layer is heated to a temperature equal to or higher than the melting point. In the region irradiated with such beam, the recording layer is melted and thereafter quenched to form an amorphous recorded mark. When the recorded mark is erased, a laser beam of relatively low power level (erasing power level) is applied so that the recording layer is heated to a temperature equal to or higher than the crystallization temperature and lower than the melting point. The recorded mark which is irradiated with such laser beam is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the phase change optical recording media, the medium can be overwritten by modulating the intensity of a single light beam.
In a phase change medium which can be rewritten by overwriting, amorphous recorded marks are formed by irradiating the crystalline recording layer with a laser beam of recording power level to melt the recording layer, and quenching the molten area to thereby form the amorphous recorded marks. In the erasing, the amorphous recorded marks are irradiated with the laser beam of erasing power level which is lower than the recording power level to heat the layer to a temperature equal to or higher than the crystallization temperature of the recording layer and lower than the melting point, and then gradually cooling the layer to thereby crystallize the amorphous recorded marks. As described above, overwriting can be accomplished by irradiating a single laser beam with its intensity modulated.
When a phase change medium is overwritten at a high speed, amorphous recorded marks should be erased (crystallized) at a high linear velocity. The rate determining factor in such process has been crystallization speed of the recording layer, namely, speed of crystalline phase conversion in the change from amorphous to crystalline phase. While overwriting at a high speed can be accomplished by using a recording layer of high crystallization speed, excessively high crystallization speed is likely to result in crystallization of the amorphous recorded marks, and hence, unstability of the medium. The medium then suffers from reduced reading stability and storage reliability. In the meanwhile, overwriting of the medium having a recording layer of low crystallization speed is associated insufficient erasing of the recorded marks as well as increase in the jitter.
A recording layer of high crystallization speed is associated with the phenomenon of selferase wherein the recorded marks partly become recrystallized in the recording due to the heat conduction in the in-plane direction as well as crosserase wherein the recorded marks on the adjacent track are erased in the recording. Both selferase and crosserase invite increase in the jitter.
As described above, it is difficult in an overwritable phase change medium to drastically increase the crystallization speed of the recording layer, and hence, the data transfer rate.
When the phase change medium is a disk-shaped medium which rotates at a constant angular velocity, the medium is associated with the problem as described below. It is to be noted that typical recording formats wherein the disk is operated at a constant angular velocity include CAV (Constant Angular Velocity) and MCAV (Modified CAV) systems. Recording formats are described, for example, in page 223 of “Optical Disk Technology” published from Radio Technology Inc. on Feb. 10, 1989.
The linear velocity of a disk-shaped medium which rotates at a constant angular velocity is slower in the radially inner side and faster in the radially outer side of the disk. On the other hand, a recording layer typically has a uniform crystallization speed over its entire area since the composition of the recording layer is uniform over its entire area. As a consequence, when the crystallization speed of the recording layer is determined so that sufficient erasability is attained at the linear velocity in the radially inner side of the medium, the disk is likely to suffer from an increased jitter in the radially outer side of the disk where the disk is operated at a higher linear velocity.